Phantom burden arrangement for current transformer calibration

ABSTRACT

The power and reactive volt-amperes normally dissipated to furnish a burden are returned to the power supply either through a current comparator or an auxiliary current transformer. This effect is achieved by the use of an amplifier connected to generate a voltage that is an adjustable complex function of the secondary current of the calibration circuit, such voltage being applied across the secondary of the transformer undergoing calibration in order to furnish an effective burden therefor.

United States Patent Kusters et a]. 1 Oct. 24, 1972 [54} PHANTOM BURDENARRANGEMENT 3,273,051 9/1966 Povey et a1. .............324/S$ X FORCURRENT TRANSFORMER 3,359,488 12/1967 Park ..324/5s CALIBRATION3,534,247 10/1970 Miljanic ..324/55 x Inventors: Norbert L. Kusters;William J. M. Moore, both of Ottawa, Ontario, Canada Canadian Patentsand Development Limited, Ottawa, Ontario, Canada Filed: Aug. 16, 1971Appl. No.: 172,093

Assignee:

US. Cl ..324/$5 Int. Cl. ..G0lr 31/06 Field of Search ..324/55References Cited UNITED STATES PATENTS 1 1/1959 Pritchett ..324/55 PoW52 5112': Y

(ween/7 Primary Examiner-Gerard R. Strecker Attorney-Richard K. Stevenset al.

[57] ABSTRACT The power and reactive volt-amperes normally dissipated tofurnish a burden are returned to the power supply either through acurrent comparator or an auxiliary current transformer. This efi'ect isachieved by the use of an amplifier connected to generate a voltage thatis an adjustable complex function of the secondary current of thecalibration circuit, such voltage being applied across the secondary ofthe transformer undergoing calibration in order to furnish an effectiveburden therefor.

8 Claims, 4 Drawing Figures A MP1 lF/EP mtmiflw m 3.701.008

sum 2 or z Pause SUPPLY +-'P l0 2/ {p 331 77 y I. I5 M 15ml Plus: r J\.SHIFTE? PHANTOM BURDEN ARRANGEMENT FOR CURRENT TRANSFORMER CALIBRATIONThis invention relates to means for realizing a given burden conditionin a current transformer calibration circuit.

Traditional burdens now used for current transformer calibrations aredissipative, often generating up to 100 watts at the rated secondarycurrent of amperes for the largest standard burdens. Present practicerequires calibration at 200 percent of rated current, and this may wellbe extended in the future to 400 percent. The correspondingdissipations, 400 watts and l ,600 watts, would be excessive.

One embodiment of the present invention described below represents animprovement over the circuits disclosed in N.L. Kusters et al US. Pat.No. 3,188,562issued June 8, 1965, and more particularly an improvementover the compensated current comparator device illustrated in FIG. 5 ofthat patent. This same patent illustrates, in FIG. 3, a phantom burdenfor use in the current calibrating circuit. Such phantom burdenrepresents an improvement over traditional burdens, in that itdissipates less power than a real burden. This prior arrangement has thedisadvantage, however, that a manual adjustment is required to achieve aburden balance.

The object of the present invention is to provide a phantom burdenarrangement for use in current transformer calibration circuits in whichthis disadvantage is avoided, while the advantage of a power dissipationlower than that of a real burden is retained.

Various embodiments of the present invention are illustrated in theaccompanying drawings and in the description that follows. Thesecircuits are provided by way of example only and not by way oflimitation of the broad scope of the present invention, which latter isdefined in the appended claims.

In the drawings:

FIG. I is a circuit diagram showing the present invention applied to acurrent transformer calibration circuit using a compensated currentcomparator device of the type illustrated in said prior patent;

FIG. la is a fragment of FIG. I showing a modification;

FIG. 2 is a circuit diagram showing the invention applied to a standardcurrent transformer calibration circuit; and

FIG. 2a is a fragment of FIG. 2 showing an alternatrve.

With reference to FIG. 1, there is shown a transformer T to becalibrated by means of a compensated current comparator device C and anassociated circuit. The primary winding of the transformer T isconnected in series with a primary winding 11 of the comparator device Cacross a power supply P. The secondary winding 12 of the transformer Tis connected in series with a secondary winding 13 of the comparatordevice C.

In addition to these windings, the comparator device C has an outer,laminated magnetic core 14, a compensation winding IS, a detectionwinding 16 and an inner, laminated magnetic core 17. The inner core 17is at the axial center of the device, the detection winding 16,compensation winding and outer core 14 being arranged radially outwardlyin that order, with the primary and secondary windings 11, 13 outsidethe core 14. All the windings generate flux in the inner core 17. sothat a zero reading on a null detector D connected across the detectionwinding 16 represents balance of the currents in the windings ll, 13 and15. On the other hand, the outer core 14 is unaffected by current in thewinding 15 which is positioned radially inwardly of it, the flux in thecore 14 thus representing the sum of the currents in the windings l land 13. The compensation winding 15 is connected between ground and apoint M between the windings l2 and 13, and must have the same number ofturns as the winding 13.

The secondary circuit 19 including the windings l2 and 13 is completedthrough primary windings 20 and 21 of a pair of small power, currenttransformers 22 and 23, respectively. The secondary 24 of thetransformer 22 provides an input to a ratio-error set 25, the output ofwhich is applied between ground and the point M.

The ratio-error set 25 may take one of various possible forms. It willbe basically an adjustable resistivereactive network for generating anerror current I, that bears a predetermined relationship to the currentin the circuit 19. One example of a ratio-error set suitable for thispurpose is illustrated in the above-mentioned patent. Alternatively,there may be used one of the ratio-error sets described in the paper ofW..I.M. Moore and N.L. Kusters entitled Direct Reading Ratio-Error Setsfor the Calibration of Current Transformers" published in the IEEETransactions on Instrumentation and Measurement, Volume IM-l9, No. 3,August I970.

The secondary 26 of the current transformer 23 is connected across aresistive burden r and also between ground and an amplifier A ofnegative a gain, this connection to the amplifier A being establishedboth as a direct, in-phase connection and through a phase shifter S toprovide a quadrature input. Gain controls GI and G2 are also included.The output of the amplifi er A acting through a transformer 27 generatesa voltage e that is a linear but complex function of the current in thewinding 21 and which is applied between ground and a point N locatedbetween the transformer windings 20 and 21. A third, feed-back input tothe amplifier A is taken from point N at the output of the amplifier. Asis known, this negative feedback manner of connection insures a verystable overall circuit gain, A1- ternatively, this third input to theamplifier A can be brought from a point Y between the windings l2 and 21(FIG. 10), greater accuracy being achieved by connecting it to the pointY. This third input represents the active voltage which is to becontrolled by the gain controls G1 and G2 and is fed back into theamplifier so that it may be compared with the desired voltage asdetermined by the settings of gain controls G1 and G2.

The primary current is assumed to be I,,, so that the secondary currentgenerated in the transformer T is I, I,., where I, is the idealsecondary current (i.e. 1,, I, as surning a nominal 1:1 ratio). l is thetransformer error current, which is the function to be determined whencalibrating the transformer T. A compensation or magnetizing currentI,,, flows in the windings 15, so that the current in the secondarywinding 13 of the comparator device C and hence in the primary winding20 of the transformer 22 is I, l,,,.

The current in the primary 21 of the current transformer 23 is I, I,, sothat, for balance at the point N, the transformer 27 must supply acurrent I,,, I, at the point N.

The impedance of the winding is low and the magnetizing current I,, issmall; hence point M is virtually at ground potential. Thus the voltagee applied between point N and ground appears across the winding 12 oftransformer I. The burden of a current transformer is by definition thevoltage across its secondary divided by the secondary current. Hence theburden on the transformer T is determined by the transfer function e/(lI,.). The power and reactive volt-amperes are returned to the supply Pvia windings 13 and 11 of the comparator device C, these windings actingwith core 14 as a transformer without affecting the detector D.

In use, the gain controls G1, G2 are set to the value of the desiredburden and the variable resistor and capacitor in the ratio-error set 25are adjusted for a null reading on the detector D. The third input tothe amplifier A is used to detect any deviation from the voltage e ofthe voltage across the winding 12, thus maintaining this latter voltageat its desired value. The settings in the ratio-error set 25 provide thedesired information concerning the magnitude and phase of the errorcurrent I,. in relation to the ideal secondary current I,.

In FIG. 2 no comparator is used; instead the winding I2 of thetransformer T to be calibrated is connected in a series secondarycircuit 19' consisting of a secondary winding 30 of a standard currenttransformer T1, a secondary winding 31 of an auxiliary currenttransformer T2 and the winding 21 of the transformer 23. The primaries32, 10 and 33 of transformers Tl, T and T2 are series connected to thepower supply P. In this case the standard and auxiliary currenttransformers TI and T2 together constitute a so-called second currenttransformer means," the first current transformer being the transformerT undergoing calibration.

The transformer 23 acts in essentially the same manner as in FIG. 1. Aresistor R in the secondary circuit 19' provides an input to theratio-error set 25 which, as before, feeds the error current I, to thepoint M. The amplifier A is also similarly connected in respect of bothits inputs and output, the point N now being located between windings 21and 31. The currents are shown on the circuit diagram. A null detector Dconnected between point M and ground is used to achieve balance.

In this embodiment the power and reactive volt-amperes are returned tothe supply P through the auxiliary current transformer T2, while thecurrent transformer T1 provides the necessary standard. Both thesefunctions were performed in FIG. 1 by the compensated current comparatordevice C.

An advantage of the FIG. 2 circuit is that the input to the ratio errorset need not necessarily be transformer coupled, as is necessary inFIG. 1. Such input can be a resistance in the secondary circuit, asshown in FIG. 2, or any other way of detecting the current in thatcircuit. This freedom enables use of any of the commercially availableratio-error sets in the FIG. 2 circuit, whereas with the FIG. I circuitit is essential to use a ratio-error set having a transformer coupledinput.

FIG. 2a shows an alternative to the FIG. 2 circuit. Instead of aratio-error set generating an error current I,.,

this current is simply allowed to flow in a direct connection betweenthe point M and the grounded loop of the secondary circuit 19'. Aresistor R1 in this connection generates a voltage Rll which voltage iscompared by detector D with a voltage generated in a voltage generator35. Adjustments in the latter will be made until the detector D readszero, whereupon the settings in the generator 35 will provide a measureof I the quantity that it is desired to determine. An input to thevoltage generator 35 from across a resistor R2 in the secondary circuit19' provides such generator with a magnitude and phase reference.

Thus the means for generating an error current that is equal to theerror current of the transformer under test (e.g. the ratio error set)has been replaced in the FIG. 2a circuit by means for generating anerror signal of a different type. In this case the error signal consistsof a voltage that is equal to the drop across a resistor produced by theerror current.

We claim:

1. Means for realizing a given burden condition in a current transformercalibration circuit including a secondary circuit containing thesecondary winding of the current transformer to be calibrated;comprising amplifier means for generating a voltage proportional to thecurrent in said secondary circuit, and means for applying said voltageacross said secondary winding to form a phantom burden arrangementtherefor.

2. Means according to claim 1, including means for controlling themagnitude and phase of said voltage in relation to the secondary circuitcurrent.

3. Means for realizing a given burden condition on a first currenttransformer in a calibration circuit, comprising a. second currenttransformer means,

b. means connecting primary windings of said first current transformerand said second current transformer means in series with each other anda power supply to form a primary circuit,

c. means connecting secondary windings of said first current transformerand said second current transformer means in series with each other toform a secondary circuit,

d. means for measuring the error current of the first transformer,

e. amplifier means for generating a voltage proportional to the currentin the secondary circuit, including means for controlling the magnitudeand phase of such voltage in relation to said secondary circuit current,

f. and means for applying said voltage across the secondary winding ofthe first transformer whereby to form a phantom burden arrangementtherefor.

4. Means according to claim 3,

g. wherein said second current transformer means (a) comprises acompensated current comparator device, and

h. wherein said measuring means (d) comprises means for injecting acurrent equal to the error current of the first current transformer intothe secondary circuit between the secondary winding of said firstcurrent transformer and a secondary winding of said current comparatordevice.

5. Means according to claim 3,

means enabling a current equal to the error current of the first currenttransformer to flow into the secondary circuit between the secondarywinding of said first current transformer and the secon dary winding ofsaid standard current transformer, and means for measuring a voltageproportional to said error current.

7. Means according to claim 3, including a feed-back input to saidamplifier means.

8. Means according to claim 7, wherein said feedback input is connectedto represent the voltage across the secondary winding of the firstcurrent transformer.

1. Means for realizing a given burden condition in a current transformercalibration circuit including a secondary circuit containing thesecondary winding of the current transformer to be calibrated;comprising amplifier means for generating a voltage proportional to thecurrent in said secondary circuit, and means for applying said voltageacross said secondary winding to form a phantom burden arrangementtherefor.
 2. Means according to claim 1, including means for controllingthe magnitude and phase of said voltage in relation to the secondarycircuit current.
 3. Means for realizing a given burden condition on afirst current transformer in a calibration circuit, comprising a. secondcurrent transformer means, b. means connecting primary windings of saidfirst current transformer and said second current transformer means inseries with each other and a power supply to form a primary circuit, c.means connecting secondary windings of said first current transformerand said second current transformer means in series with each other toform a secondary circuit, d. means for measuring the error current ofthe first transformer, e. amplifier means for generating a voltageproportional to the current in the secondary circuit, including meansfor controlling the magnitude and phase of such voltage in relation tosaid secondary circuit current, f. and means for applying said voltageacross the secondary winding of the first transformer whereby to form aphantom burden arrangement therefor.
 4. Means according to claim 3, g.wherein said second current transformer means (a) comprises acompensated current comparator device, and h. wherein said measuringmeans (d) comprises means for injecting a current equal to the errorcurrent of the first current transformer into the secondary circuitbetween the secondary winding of said first current transformer and asecondary winding of said current comparator device.
 5. Means accordingto claim 3, g. wherein said second current transformer means (a)comprises a standard current transformer and an auxiliary currenttransformer, and h. wherein said measuring means (d) comprises means forinjecting a current equal to the error current of the first currenttransformer into the secondary circuit between the secondary winding ofsaid first current transformer and the secondary winding of saidstandard current transformer.
 6. Means according to claim 3, g. whereinsaid second current transformer means (a) comprises a standard currenttransformer and an auxiliary current transformer, and h. wherein saidmeasuring means (d) comprises means enabling a current equal to theerror current of the first current transformer to flow into thesecondary circuit between the secondary winding of said first currenttransformer and the secondary winding of said standard currenttransformer, and means for measuring a voltage proportional to saiderror current.
 7. Means according to claim 3, including a feed-backinput to said amplifier means.
 8. Means according to claim 7, whereinsaid feed-back input is connected to represent the voltage across thesecondary winding of the first current transformer.